1. Field of the Invention
The present invention relates to an optical disc apparatus and data writing method for minimizing the resonance (or mechanical vibrations) of an optical disc such as a DVD by detecting the magnitude of the resonance produced and by changing the number of revolutions of the optical disc before the magnitude of the resonance exceeds a predetermined value.
2. Description of the Related Art
In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity and detecting the light that has been modulated by, and reflected from, the optical disc. A unit that emits such a light beam and detects the light reflected from an optical disc is called an “optical pickup”. An optical disc is rotated by a motor.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits and tracks and the thickness of the recording material film are both smaller than the thickness of the optical disc substrate. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane” or an “information plane”. However, considering that such a plane actually has a physical dimension in the depth direction, too, the term “storage plane (or information plane)” will be replaced herein by another term “information storage layer”. Every optical disc has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
In a recordable or rewritable optical disc, when data is going to be written on its information storage layer, the information storage layer is irradiated with such a light beam, of which the optical power has been modulated as described above, thereby forming an amorphous recorded mark on a crystalline phase change material layer. Such an amorphous recorded mark is left there by heating a portion of the information storage layer that has been irradiated with a writing light beam to a temperature that is equal to or higher than its melting point and then rapidly cooling that portion. If the optical power of a light beam that irradiates the recorded mark is set to be relatively low, the temperature of the recorded mark being irradiated with the light beam does not exceed its melting point but the recorded mark will turn crystalline again after having been cooled rapidly (i.e., the recorded mark will be erased). In this manner, the recorded mark can be rewritten over and over again. However, if the optical power of the light beam for writing data (i.e., optical recording power) had an inappropriate level, then the recorded mark would have a deformed shape and sometimes it could be difficult to read the data as intended.
Such an amorphous recorded mark has a different reflectance from its surround crystalline portions. For that reason, when a read operation is performed, the intensity of the reflected light varies depending on whether or not a recorded mark is there. In an area where data has already been written (which will be referred to herein as a “recorded area”), there is a series of recorded marks and spaces, of which the lengths are variable with the contents of the data to be written. For that reason, the optical properties (i.e., the optical reflectance and transmittance) of such a recorded area are different from those of an area where no data has been written yet (which will be referred to herein as an “unrecorded area”).
To read data that is stored on an optical disc or to write data on a rewritable optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” need to be done. The “focus control” means controlling the position of an objective lens along a normal to the surface of the information plane (such a direction will sometimes be referred to herein as “substrate depth direction”) so that the focal point (or at least the converging point) of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as an “optical disc radial direction”) such that the light beam spot is always located right on a target track.
In order to perform such a focus control or a tracking control, the focus error or the tracking error needs to be detected based on the light that has been reflected from the optical disc and the position of the light beam spot needs to be adjusted so as to reduce the error as much as possible. The magnitudes of the focus error and the tracking error are represented by a “focus error (FE) signal” and a “tracking error (TE) signal”, both of which are generated based on the light that has been reflected from the optical disc.
An optical disc apparatus for writing information on a given optical disc such as a DVD or a Blu-ray Disc (BD) and reading the information stored on it will sometimes be prevented from performing a write operation with good stability and quality by the eccentricity or out-of-plane vibrations (which is also called “disc flutter”) of the optical disc. As used herein, the “eccentricity of an optical disc” refers to a situation where the center (of mass) of a given optical disc is not aligned with the center of rotation of that optical disc being turned by a motor. The greater the magnitude of eccentricity of a given optical disc, the more significantly the center of mass of that disc moves during its rotation. As a result, mechanical vibrations will be produced in such a situation. On the other hand, the “out-of-plane vibrations” will be produced if the surface of a given optical disc is not quite perpendicular to the center of rotation of that disc being turned by a motor. In that case, as the optical disc rotates, the interval between the optical pickup and the surface of the optical disc will vary periodically. Nevertheless, even if the given optical disc has eccentricity or produces out-of-plane vibrations, the light beam can still follow the target tracks on the target information storage layer of that optical disc by performing the focus control or tracking control described above, unless the degree of such eccentricity or out-of-plane vibrations is excessive.
A conventional optical disc apparatus (which will be sometimes referred to herein as an “optical drive”) includes a signal processor for determining whether the write quality of a read signal that has been obtained by an optical pickup is good or bad and for removing signal components representing the optical disc's eccentricity or out-of-plane vibrations from that read signal. Meanwhile, as there is a growing demand for personal computers (PCs) of even smaller sizes, the overall sizes and thicknesses of optical drives have been further reduced lately.
Japanese Patent Applications Laid-Open Publications Nos. 2006-252767, 8-55422, 5-347068, 6-44595, 2-94063 and 2002-109822 disclose techniques for decreasing the number of revolutions of an optical disc when the level of a servo signal or a data read error exceeds a reference level.
The driving force of conventional optical pickups was high enough to keep up with the eccentricity of a given optical disc. Lately, however, as the overall sizes or thicknesses of optical drives have been reduced, the driving force of recent optical pickups have become too low to keep up with the resonances to be produced by the eccentricity of a given optical disc.
An optical disc apparatus (decoder) that can read and write from/to both DVDs and BDs usually has two objective lenses for DVDs and BDs, respectively. If an optical pickup were designed with the objective lens for DVDs prioritized, then the center of mass of the optical pickup would lose balance when a read/write operation is performed using an objective lens for BDs. That is why when a so-called “slim drive” with two lenses performs a read/write operation using an objective lens for BDs, the resonances produced due to the eccentricity of an optical disc are no longer negligible. As used herein, the “slim drive” refers to an optical disc drive with a thickness of 12.7 mm (=0.5 inches) or less. In a slim drive, the optical pickup to use also needs to have a reduced size. However, if resonances were produced due to the eccentricity of the optical disc being rotated, it would be difficult, more often than not, for such an optical pickup of a reduced size for a slim drive to direct a light beam toward a target location on the optical disc.
If resonances were produced due to the eccentricity of an optical disc, a recorded mark formed on an information storage layer of the optical disc could have a deformed shape or have its location shifted from the target one. And if data were written in such an unintentional state, a read error should occur when such data is read.
It is therefore an object of the present invention to provide an optical disc apparatus and data writing method that can minimize such resonances while a given optical disc is rotating.